Method and system for detecting LED short circuit in LED strings or detecting matching among LED strings

ABSTRACT

This specification relates to a method and a system for detecting light emitting diode (LED) short circuit in a plurality of LED strings or detecting matching among the plurality of LED strings, wherein one end of each of the plurality of LED strings is connected to a same output end of a power supply, and the other end is respectively connected to a corresponding switch. According to the present invention, first current of each of LED strings is obtained when the output end of the power supply outputs a first voltage; differences between the minimum of the first currents of LED strings and other first currents are calculated; the differences are compared with a comparing threshold; it is determined that the LED strings corresponding to the other currents for which differences are larger than the comparing threshold include short circuit or mismatch with the LED string corresponding to the minimum current. Accordingly, it is possible to reduce the number of pins and area of a control chip.

RELATED APPLICATION

This application claims priority to Chinese Patent Application No.2013100864061 filed Mar. 18, 2013, which is incorporated by referenceherein in its entirety.

TECHNICAL FIELD

Embodiments of the present invention relate to the field of LED drive,and more specifically to a method and a system for detecting lightemitting diode (LED) short circuit in a plurality of LED strings ordetecting matching among the plurality of LED strings.

BACKGROUND

A light emitting diode (briefly called LED) is extensively applied tomany environments such as building lighting, vehicle head lamps and taillamps, backlights and flash lights of liquid crystal display devicesincluding personal computers and high-definition television sets. Ascompared with conventional light sources such as incandescent lamps andfluorescent lamps, LEDs have distinctive advantages such highefficiency, good directivity, excellent color stability, highreliability, long service life, small size and safe environment.

Generally, a plurality of LEDs are arranged into an LED string, and inmany environments, for example, a plurality of LED strings are used inparallel in backlights and flash lights of liquid crystal displaydevices including personal computers and high-definition televisionsets.

In use of the LEDs, the corresponding LEDs in each LED string might beshort circuited.

Hence, it is desirable to timely, correctly and conveniently detect theshort circuit of LEDs in the LED string.

FIG. 1 is a schematic view illustrating detection of LED short circuitin the prior art. For the purpose of brevity, only one LED string 101 isshown. In this prior art, a voltage at a drain of an MOSFET of a switch102 controlling turn-on or turn-off of the LED string is directlydetected and compared with a comparison threshold. If the voltage on thedrain is higher than the comparison threshold, it indicates that thereare short circuited LEDs in the LED string; if the voltage on the drainis lower than the comparison threshold, it indicates that there are noshort circuited LEDs in the LED string.

However, the MOSFET drain is a high-voltage pin, and sometimes itsvoltage will reach 100 V. Hence, an area of a chip is substantiallyexpanded if units for detecting short circuit of LEDs in the LED stringare integrated in a control chip 103. Besides, since there are aplurality of LED strings and each LED string needs to be detected to seewhether the LEDs therein are short circuited, the number of pins of thecontrol chip 103 will be increased by detecting the voltage on theMOSFET drain to detect whether there are short circuited LEDs in thecorresponding LED strings.

Additionally, due to manufacture differences among LEDs, for the samecurrent, the voltage drops are rather different among the LED strings.In other words, for the same current output, currents might be ratherdifferent among the LED strings. This might cause a wrong result indetecting short circuit of LEDs in the LED string. Therefore, before theLED strings are applied to specific environments, it is optimal toperform match detection of the LED strings and to apply matching LEDstrings, namely, LED strings whose voltage drop difference is less thana comparison threshold, to the same specific environment.

SUMMARY

The disclosure is intended to provide a method and a system fordetecting a light emitting diode (LED) short circuit in a plurality ofLED strings or detecting matching among the plurality of LED strings,which can address the above-mentioned problems existing in the priorart.

According to one aspect of the present invention, there is provided asystem for detecting light emitting diode (LED) short circuit in aplurality of LED strings or detecting matching among the plurality ofLED strings, wherein one end of each of the plurality of LED strings isconnected to a same output end of a power supply, and the other end isrespectively connected to a corresponding switch, the system comprising:a plurality of first current obtaining units, configured to obtain firstcurrents in the LED strings respectively when the output end of thepower supply outputs a first voltage; a calculating unit, configured tocalculate differences between the minimum of the first currents in eachof the LED strings and other first currents; a comparing unit,configured to compare the differences with a comparing threshold; and adetermining unit, configured to determine the LED strings correspondingto those other currents for which differences are larger than thecomparing threshold to have LED short circuit or mismatch with the LEDstring corresponding to the minimum current.

According to another aspect of the present invention, there is provideda system for detecting light emitting diode (LED) short circuit in aplurality of LED strings or detecting matching among the plurality ofLED strings, wherein one end of each of the plurality of LED strings isconnected to a same output end of a power supply, and the other end isrespectively connected to corresponding switch, the system comprising: aplurality of first current obtaining units, configured to obtain firstcurrents in the LED strings respectively when the output end of thepower supply outputs a first voltage; a changing unit, configured tochange the output at the output end of the power supply to a secondvoltage; a plurality of second current obtaining units, configured toobtain second currents in the LED strings respectively when the outputend of the power supply outputs the second voltage; a determining unit,configured to determine whether there is LED short circuit in the LEDstrings or whether the LED strings matches with one another according tothe obtained first and second currents in the LED strings.

According to a further aspect of the present invention, there isprovided a method for detecting light emitting diode (LED) short circuitin a plurality of LED strings or detecting matching among the pluralityof LED strings, wherein one end of each of the plurality of LED stringsis connected to a same output end of a power supply, and the other endis respectively connected to a corresponding switch, the methodcomprising: obtaining first currents in the LED strings respectivelywhen the output end of the power supply outputs a first voltage;calculating differences between the minimum of the first currents in theLED strings and other first currents; comprising the differences with acomparing threshold; and determining LED strings corresponding to thoseother currents for which differences are larger than the comparingthreshold to have LED short circuit or mismatch with the LED stringcorresponding to the minimum current.

According to another aspect of the present invention, there is provideda method for detecting light emitting diode (LED) short circuit in aplurality of LED strings or detecting matching among the plurality ofLED strings, wherein one end of each of the plurality of LED strings isconnected to a same output end of a power supply, and the other end isrespectively connected to a corresponding switch, the method comprising:obtaining first currents in the LED strings respectively when the outputend of the power supply outputs a first voltage; changing output at theoutput end of the power supply to a second voltage; obtaining secondcurrents in the LED strings respectively when the output end of thepower supply outputs the second voltage; determining whether there isLED short circuit in the LED strings or whether the LED strings matchwith one another according to the obtained first and second currents inthe LED strings.

According to a further aspect of the present invention, there isprovided a system for detecting light emitting diode (LED) short circuitin a plurality of LED strings or detecting matching among the pluralityof LED strings, wherein one end of each of the plurality of LED stringsis connected to a same output end of a power supply, and the other endis respectively connected to a corresponding switch, the systemcomprising: a plurality of first voltage obtaining units, configured toobtain first voltages at control ends of the corresponding switchesrespectively when each of the LED strings has a same current; acalculating unit, configured to calculate differences between themaximum of the first voltages at the control ends of the correspondingswitches and other first voltages; a comparing unit, configured tocompare the differences with a comparing threshold; and a determiningunit, configured to determine the LED strings corresponding to thoseother voltages for which differences are larger than the comparingthreshold to have LED short circuit or mismatch with the LED stringcorresponding to the maximum voltage.

According to a further aspect of the present invention, there isprovided a system for detecting light emitting diode (LED) short circuitin a plurality of LED strings or detecting matching among the pluralityof LED strings, wherein one end of each of the plurality of LED stringsis connected to a same output end of a power supply, and the other endis respectively connected to a corresponding switch, the systemcomprising: a plurality of first voltage obtaining units, configured toobtain first voltages at control ends of the corresponding switchesrespectively when each of the LED strings has same current and theoutput end of the power supply outputs a first voltage; a changing unit,configured to change the output at the output end of the power supply toa second voltage; a plurality of second voltage obtaining units,configured to obtain second voltages at the control ends of thecorresponding switches respectively when the output end of the powersupply outputs the second voltage; a determining unit, configured todetermine whether there is LED short circuit in the LED strings orwhether the LED strings match with one another according to the obtainedfirst and second voltages at the control ends of the correspondingswitches.

According to a further aspect of the present invention, there isprovided a method for detecting light emitting diode (LED) short circuitin a plurality of LED strings or detecting matching among the pluralityof LED strings, wherein one end of each of the plurality of LED stringsis connected to a same output end of a power supply, and the other endis respectively connected to a corresponding switch, the methodcomprising: obtaining first voltages at control ends of thecorresponding switches respectively when each of the LED strings has asame current; calculating differences between the maximum of the firstvoltages at the control ends of the corresponding switches and othervoltages; comparing the differences with a comparing threshold; anddetermining the LED strings corresponding to those other voltages forwhich differences are larger than the comparing threshold to have LEDshort circuit or mismatch with a LED string corresponding to the maximumvoltage.

According to a further aspect of the present invention, there isprovided a method for detecting light emitting diode (LED) short circuitin a plurality of LED strings or detecting matching among the pluralityof LED strings, wherein one end of each of the plurality of LED stringsis connected to a same output end of power supply, and the other end isrespectively connected to a corresponding switch, the method comprising:obtaining first voltages at control ends of the corresponding switchesrespectively when each of the LED strings has a same current and theoutput end of the power supply outputs a first voltage; changing theoutput at the output end of the power supply to a second voltage;obtaining second voltages at the control ends of the correspondingswitches respectively when the output end of the power supply outputsthe second voltage; determining whether there is LED short circuit inthe LED strings or whether the LED strings match with one anotheraccording to the obtained first and second voltages at the control endof the corresponding switches.

According to the present invention, it is possible to reduce the numberof pins and area of a control chip because there is no need to detectthe voltage of a switch terminal which generally exhibits a highvoltage, wherein the switch terminal needn't be connected to the controlchip in the case of not aiming to detect whether the LED strings haveLED short circuit.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The figures described here are used to provide further understanding ofthe present application and constitute one part of the presentapplication. Illustrative embodiments of the present invention anddepictions thereof are used to illustrate the present invention and donot constitute improper limitations of the present invention. In thefigures,

FIG. 1 is a schematic view illustrating detection of LED short circuitaccording to a conventional system.

FIG. 2 illustrates an exemplary environment in which the presentinvention may be implemented.

FIG. 3 illustrates a block diagram of a system for detecting LED shortcircuit in a plurality of LED strings or detecting matching among theplurality of LED strings according to an embodiment of the presentinvention.

FIG. 4 illustrates a schematic view of a first current obtaining unitaccording to an embodiment of the present invention.

FIG. 5 illustrates a block diagram of a system for detecting LED shortcircuit in a plurality of LED strings or detecting matching among theplurality of LED strings according to another embodiment of the presentinvention.

FIG. 6 illustrates a flow chart of a method for detecting LED shortcircuit in a plurality of LED strings or detecting matching among theplurality of LED strings according to an embodiment of the presentinvention.

FIG. 7 illustrates a flow chart of a method for detecting LED shortcircuit in a plurality of LED strings or detecting matching among theplurality of LED strings according to another embodiment of the presentinvention.

FIG. 8 illustrates a block diagram of a system for detecting LED shortcircuit in a plurality of LED strings or detecting matching among theplurality of LED strings according to a further embodiment of thepresent invention.

FIG. 9 illustrates a schematic view of a current arranging unitaccording to an embodiment of the present invention.

FIG. 10 illustrates a block diagram of a system for detecting LED shortcircuit in a plurality of LED strings or detecting matching among theplurality of LED strings according to another embodiment of the presentinvention.

FIG. 11 illustrates a flow chart of a method for detecting LED shortcircuit in a plurality of LED strings or detecting matching among theplurality of LED strings according to an embodiment of the presentinvention.

FIG. 12 illustrates a flow chart of a method for detecting LED shortcircuit in a plurality of LED strings or detecting matching among theplurality of LED strings according to another embodiment of the presentinvention.

In the figures, the same reference numbers denote the same or likefeatures or functions.

DETAILED DESCRIPTION

Noticeably, in the absence of conflicts, embodiments in the presentapplication and features in the embodiments may be combined with oneanother. The present invention will be described in detail incombination with embodiments with reference to the figures.

FIG. 2 illustrates an exemplary environment in which the presentinvention may be implemented. It will be described in detail hereunder.

An environment 200 comprises a power supply 243, LED strings 242-1,242-2 and 242-3, switches 241-1, 241-2 and 241-3, and resistors R1, R2and R3.

The power supply 243 is for example a boost converter and connected toone end of each of the LED strings 242-1, 242-2 and 242-3 to provide anoutput voltage to the LED strings 242-1, 242-2 and 242-3. In embodimentsof the present invention, the output voltage of the power supply 243 isvariable, i.e., it may rise or fall.

Certainly, those skilled in the art may understand that the power supply243 may also be a buck converter or a buck-boost converter.

The other ends of the LED strings 242-1, 242-2 and 242-3 are connectedto the corresponding switches 241-1, 241-2 and 241-3, respectively. Theswitches 241-1, 241-2 and 241-3 may be either an MOSFET, or a bipolartransistor. Here, without loss of generality, assume the switches 241-1,241-2 and 241-3 to be MOSFETs.

More specifically, the other ends of the LED strings 242-1, 242-2 and242-3 are connected to drains of the switches 241-1, 241-2 and 241-3respectively. And sources of the switches 241-1, 241-2 and 241-3 aregrounded via the resistors R1, R2 and R3, respectively.

A control signal (not shown in FIG. 2) may be applied respectively togates of the switches 241-1, 241-2 and 241-3, to control ON and OFF ofthe switches 241-1, 241-2 and 241-3, such that the ON and OFF of the LEDstrings 242-1, 242-2 and 242-3 may be controlled.

In the following description, for the purpose of brevity, each of theLED strings 242-1, 242-2 and 242-3 is assumed to have the same number ofLEDs.

Noticeably, the number of LED strings according to the present inventionis not limited to three. The present invention is also adapted forsituations in which the number of LED strings is greater than three orless than three, e.g., two, four, five or six.

FIG. 3 illustrates a block diagram of a system for detecting LED shortcircuit in three LED strings shown in FIG. 2 or detecting matching amongthe three LED strings shown in FIG. 2 according to an embodiment of thepresent invention.

As shown in FIG. 3, the system 300 comprises three first currentobtaining units 301-1, 301-2 and 301-3 for obtaining a respective firstcurrent of the LED strings 242-1, 242-2 and 242-3 when the power supply243 outputs a first voltage;

a calculating unit 302 configured to calculate differences between theminimum of the first currents in each of the LED strings 242-1, 242-2and 242-3 and other first currents;

a comparing unit 303 configured to compare the differences with acomparing threshold; and

a determining unit 304 configured to determine the LED stringscorresponding to those other currents for which differences are largerthan the comparing threshold to have LED short circuit or mismatch withthe LED string corresponding to the minimum current.

Wherein, a magnitude of the comparing threshold corresponds to thenumber of short circuited LEDs to be detected or to unmatchingstandards. For example, the above comparing threshold may be set todetect short circuit of one LED, two LEDs or three LEDs. If the abovecomparing threshold is set to detect short circuit of three LEDs,situations in which one or two LEDs are short circuited are consideredas none occurrence of short circuit.

For example, given that the first current of the LED string 242-1 is 110mA, the first current of the LED string 242-2 be 120 mA, and the firstcurrent of the LED string 242-3 be 130 mA.

It can be believed that the LED string corresponding to the minimumcurrent among the first currents, namely, the LED string 242-1, is notshort circuited and regarded as a reference for detecting match of theLED strings. Upon knowing general volt-ampere characteristics of eachLED and knowing the number of LEDs in each LED string and the outputvoltage of the power supply, the comparing threshold may be set. Forexample, without loss of generality, the comparing threshold is 15 mA.

As such, the difference between the first currents of the LED string242-1 and LED string 242-2 is 10 mA, less than the threshold 15 mA, soit is believed that the LED string 242-2 does not include shortcircuited LED, and the LED string 242-1 matches the LED string 242-2. Incontrast, the difference between the first currents of the LED string242-1 and LED string 242-3 is 20 mA, larger than threshold 15 mA, so itis believed that the LED string 242-3 includes short circuited LEDs, andthe LED string 242-1 does not match the LED string 242-3.

First current obtaining units 301-1, 301-2 and 301-3 obtain the firstcurrents of the respective LED strings in a way of being at the same orsubstantially same biasing voltage at the gate of the correspondingMOSFET.

According to an embodiment of the present invention, during theobtainment of the first currents of the respective LED strings, thebiasing voltage is fixed and invariable, for example, at 5 V. Under thebiasing voltage, the corresponding MOSFET is completely on, and animpedance value between its drain and source is very low.

In this embodiment, the first current obtaining units 301-1, 301-2 and301-3 obtain the first currents of the respective LED strings bysampling the voltage of the source of the corresponding MOSFET.

Besides, according to one embodiment of the present invention, thesystem 300 may further comprise an analog-digital converter (not shownin FIG. 3) to perform analog-digital conversion for the sampled voltage.

Since the sample voltage is relatively small (e.g., 0-1V), to provide acorrect current, the analog-digital conversion may be performed for thesample voltage to obtain for example eight bits, so as to represent onevalue in a range of 0-255 mA.

According to an embodiment of the present invention, during theobtainment of the first currents of the respective LED strings, thebiasing voltage is controllably variable, wherein the biasing voltagevaries in a way that the currents of the LED strings are set, whereinthe first current of each of the LED strings is the maximum currentamong the set currents satisfying the biasing voltage under one biasingvoltage threshold or between two biasing voltage thresholds.

For the purpose of brevity, FIG. 4 shows the first current obtainingunit 301-1, the biasing voltage of which is controllably variable duringthe obtainment of the first currents of the respective LED strings.

The first current obtaining units 301-2 and 301-3 are structurallysimilar.

Specifically, the first current obtaining unit 301-1 comprises a firstoperational amplifier 4122 in which the inverting input terminal isconnected to the gate of the switch 241-1, and the non-inverting inputterminal is connected to the biasing threshold (e.g., 4.5 V); a firstcurrent digital-analog converter 4123, which is controlled by the outputof the first operational amplifier 4122; and a second operationalamplifier 4124 in which the non-inverting input terminal is connected toan output terminal of the first digital-analog converter 4123, and theinverting input terminal is connected to a source of the switch 241-1,and the output terminal is connected to a gate of the switch 241-1.

First, a value of the first current digital-analog converter 4123 isset, wherein the current of the LED string 242-1 corresponds to thevalue of the first current digital-analog converter 4123, the output ofthe first current digital-analog converter 4123 is set, i.e., thecurrent of the LED string 242-1 is set, and under the value, it isdetermined whether the voltage of the gate of the switch 241-1 isgreater than the biasing voltage threshold (for example, when thevoltage at the gate of the switch 241-1 is greater than the biasingvoltage threshold, the first operational amplifier 4122 outputs 0; whenthe voltage at the gate of the switch 241-1 is less than the biasingvoltage threshold, the first operational amplifier 4122 outputs 1). Ifthe voltage of the gate is not greater than the biasing voltagethreshold, the value outputted by the first current digital-analogconverter 4123 is increased (because the increase of the value outputtedby the first current digital-analog converter 4123 will cause anincrease of the voltage of the gate of the switch 241-1), and then,under the new value, it is determined whether the voltage of the gate ofthe switch 241-1 is greater than the biasing voltage threshold. Theabove procedure is continued until the voltage of the gate of the switch241-1 is equal to or substantially equal to the biasing voltagethreshold. The maximum current among the set currents with the biasingvoltage being under a biasing voltage threshold is taken as the firstcurrent of each of the LED strings.

Besides, although not shown in FIG. 4, those skilled in the art mayunderstand that the first current obtaining unit 301-1 may furthercomprise another operational amplifier in which an input terminal isconnected to the gate of the switch 241-1, the other input terminal isconnected to another biasing voltage threshold, e.g., slightly smallerthan the above biasing voltage threshold, for example, 4V, and theoutput terminal controls the first current digital-analog converter 4123for setting the output of the first current digital-analog converter4123. In such embodiment, the first current of each of the LED stringsis the maximum current among the set currents satisfying that thebiasing voltage of the gate is between two biasing voltage thresholds.

Noticeably, the above embodiment only exemplarily describes anembodiment for obtaining the current through the LED string 242-1. Inactual application, other embodiments which can achieve this function oreffect all should fall within the protection scope of the presentinvention.

Under the circumstances that the comparing threshold cannot be set, forexample, since the general volt-ampere characteristics of each LED arenot known, it is impossible to detect LED short circuit in a pluralityof LED strings or detect matching among the plurality of LED strings byusing the system 300 as described with reference to FIG. 3.

FIG. 5 illustrates a block diagram of a system for detecting LED shortcircuit in three LED strings shown in FIG. 2 or detecting matching amongthe three LED strings shown in FIG. 2 according to an embodiment of thepresent invention. The system 500 can address the problem that cannot besolved by the above system 300.

As shown in FIG. 5, the system 500 comprises: three first currentobtaining units 501-1, 501-2 and 501-3 to respectively obtain firstcurrents of the LED strings 242-1, 242-2 and 242-3 when the power supply243 outputs a first voltage; a changing unit 502, configured to changethe output at the output end of the power supply 243 to a secondvoltage; three second current obtaining units 503-1, 503-2 and 503-3,configured to obtain second currents in each of the LED strings 242-1,242-2 and 243-3 when the output end of the power supply 243 outputs asecond voltage; a determining unit 504, configured to determine whetherthere is LED short circuit in each of the LED strings 242-1, 242-2 and242-3 or whether each of the LED strings matches with each otheraccording to the obtained first and second currents in each of the LEDstrings 242-1, 242-2 and 242-3.

The first current obtaining units 501-1, 501-2 and 501-3 may beidentical with the second current obtaining units 503-1, 503-2 and503-3, and identical with the aforesaid first current obtaining units301-1, 301-2 and 301-3.

Besides, noticeably, there are two manners for obtaining the firstcurrent as stated above. Preferably, the manner for obtaining the secondcurrent corresponds to the manner for obtaining the first current.

The second voltage may be higher than the first voltage, or lower thanthe first voltage, and the difference between the second voltage and thefirst voltage may be related to for example the number of LEDs for whichshort circuit is to be detected. Under the circumstance that the secondvoltage is higher than the first voltage, the determining unit 504determines that the LED string having the first current larger than theminimum current in the second currents of the respective LED strings hasLED short circuit or mismatches.

For example, given that the first voltage is 60 V, the first current ofthe LED string 242-1 is 100 mA, the first current of the LED string242-2 is 120 mA, the first current of the LED string 242-3 is 150 mA;when the second voltage is 66V, the second current of the LED string242-1 is 130 mA, the second current of the LED string 242-2 is 160 mA,the second current of the LED string 242-3 is 200 mA. Then, it isdetermined that the LED string 242-3 has LED short circuit or mismatchesthe LED strings 242-1 and 242-2 because its current at the smaller firstvoltage is larger than the current of the LED string 242-1 at a largersecond voltage. If there is no LED short circuit, this situation issubstantially impossible.

Likewise, when the second voltage is lower than the first voltage, thedetermining unit 504 determines that the LED string having the secondcurrent larger than the minimum circuit among the first currents of therespective LED strings has LED short circuit or mismatches.

For example, given that first voltage is 60 V, the first current of theLED string 242-1 is 100 mA, the first current of the LED string 242-2 is120 mA, the first current of the LED string 242-3 is 150 mA; when thesecond voltage is 54V, the second current of the LED string 242-1 is 75mA, the second current of the LED string 242-2 is 90 mA, the secondcurrent of the LED string 242-3 is 110 mA. Then, it is determined thatthe LED string 242-3 has LED short circuit or mismatches the LED strings242-1 and 242-2 because its current at the smaller second voltage islarger than the current of the LED string 242-1 at the larger firstvoltage. If there is no LED short circuit, this situation issubstantially impossible.

The above systems 300 and 500 may be implemented by using hardware(e.g., control chip), or by using software (e.g., computer programcode), or even by using software in combination with hardware.

FIG. 6 illustrates a flow chart of a method for detecting LED shortcircuit in a plurality of LED strings or detecting matching among theplurality of LED strings according to an embodiment of the presentinvention, wherein one end of each of the plurality of LED strings isconnected to a same output end of a power supply, and the other end isrespectively connected to a corresponding switch.

As shown in FIG. 6, the method 600 comprises step 610 for obtainingfirst currents in each of the LED strings when the output end of thepower supply outputs a first voltage; step 620 for calculatingdifferences between the minimum current of the first currents in each ofthe LED strings and other first currents; step 630 for comparing thedifferences with a comparing threshold; and step S640 for determiningLED strings corresponding to those other currents for which differencesare larger than the comparing threshold to have LED short circuit ormismatch with the LED string corresponding to the minimum current.

FIG. 7 illustrates a flow chart of a method for detecting LED shortcircuit in a plurality of LED strings or detecting matching among theplurality of LED strings according to another embodiment of the presentinvention, wherein one end of each of the plurality of LED strings isconnected to a same output end of a power supply, and the other end isrespectively connected to a corresponding switch.

As shown in FIG. 7, the method 700 comprises step S710 for obtainingfirst currents in each of LED strings when the output end of the powersupply outputs a first voltage; step S720 for changing output at theoutput end of the power supply to a second voltage; step S730 forobtaining second currents in each of the LED strings when the output endof the power supply outputs a second voltage; step S740 for determiningwhether there is LED short circuit in each of the LED strings or whethereach of the LED strings matches with each other according to theobtained first and second currents in each of the LED strings.

For the purpose of brevity, in the methods as described in FIG. 6 andFIG. 7, it is assumed that the number of LEDs of each LED string is thesame.

According to an embodiment of the present invention, in the methoddescribed in FIG. 6 and FIG. 7, the corresponding switch isMOSFET/bipolar transistor, and said other end is respectively connectedto the drain of the corresponding MOSFET/a collector of the bipolartransistor.

According to an embodiment of the present invention, in the methoddescribed in FIG. 6 and FIG. 7, the first current and second current ofthe respective LED strings are obtained in a way that the gate of thecorresponding MOSFET/a base of the bipolar transistor is under the samebiasing voltage.

According to an embodiment of the present invention, in the methoddescribed in FIG. 6 and FIG. 7, during the obtainment of the firstcurrent and second current of the respective LED strings, the biasingvoltage remains invariable, and the first current and second current ofthe respective LED strings are obtained by sampling a voltage at asource of the corresponding MOSFET/an emitter of the bipolar transistor.

According to an embodiment of the present invention, in the methoddescribed in FIG. 6 and FIG. 7, the method further comprises a step ofperforming analog-digital conversion for the sampled voltage (not shownin the figures).

According to an embodiment of the present invention, in the methoddescribed in FIG. 6 and FIG. 7, during the obtainment of the firstcurrent and second current of the respective LED strings, the biasingvoltage is controllably variable, wherein the biasing voltage varies ina way that the currents of the LED strings are set, wherein the firstcurrent and second current of each of the LED strings is the maximumcurrent among the set currents satisfying the biasing voltage under onebiasing voltage threshold or between two biasing voltage thresholds.

According to an embodiment of the present invention, in the methoddescribed in FIG. 7, the second voltage may be higher than the firstvoltage, or lower than the first voltage. Under the circumstance thatthe second voltage is higher than the first voltage, the LED stringhaving the first current larger than the minimum current in the secondcurrents of the respective LED strings is determined to have LED shortcircuit or mismatch. Under the circumstance that the second voltage islower than the first voltage, the LED string having the second currentlarger than the minimum current in the first currents of the respectiveLED strings is determined to have LED short circuit or mismatch.

According to an embodiment of the present invention, in the methoddescribed in FIG. 6, a magnitude of the comparing threshold correspondsto the number of short circuited LEDs to be detected or to unmatchingstandards.

In all the above embodiments, by setting the biasing voltage at thegates of the respective switches to be identical, whether the LEDs ineach LED string have short circuit or whether all LED strings match isdetermined by measuring the current (the first current or the secondcurrent) in respective LED string.

In all the following embodiments, by setting the current in respectiveLED string to be identical, whether the LEDs in each LED string haveshort circuit or whether all LED strings match is determined bymeasuring the voltage (the first voltage or second voltage) of the gatesof the corresponding MOSFETs of all LED strings.

FIG. 8 illustrates a block diagram of a system for detecting LED shortcircuit in three LED strings shown in FIG. 2 or detecting matching amongthe three LED strings shown in FIG. 2 according to an embodiment of thepresent invention.

As shown in FIG. 8, the system 800 comprises three first voltageobtaining units 801-1, 801-2, 801-3 for obtaining first voltages atcontrol ends of the corresponding switches 241-1, 241-2, 241-3 when eachof the LED strings 242-1, 242-2, 242-3 has same current;

a calculating unit 802, configured to calculate differences between themaximum of the first voltages at the control ends of the correspondingswitches 241-1, 241-2, 242-3 and other first voltages;

a comparing unit 803, configured to compare the differences with acomparing threshold; and

a determining unit 804, configured to determine the LED stringscorresponding to those other voltages for which differences are largerthan the comparing threshold to have LED short circuit or mismatch withthe LED string corresponding to the maximum voltage.

A magnitude of the comparing threshold corresponds to the number ofshort circuited LEDs to be detected or to unmatching standards. Forexample, the above comparing threshold may be set to detect shortcircuit of one LED, two LEDs or three LEDs. If the above comparingthreshold is set to detect short circuit of three LEDs, situations inwhich one or two LEDs are short circuited are considered as noneoccurrence of short circuit.

For example, given that the first voltage of the gate of the MOSFET241-1 of the LED string 242-1 is 4.4V, the first voltage of the gate ofthe MOSFET 241-2 of the LED string 242-2 is 4.2V, and the first voltageof the gate of the MOSFET 241-3 of the LED string 242-3 is 4.0 V.

It can be believed that the LED string corresponding to the maximumvoltage among the first voltages of the gates, namely, the LED string242-1, is not short circuited and regarded as a reference for detectingmatching of the LED strings. Upon knowing general volt-amperecharacteristics of each LED and knowing the number of LEDs in each LEDstring and the output voltage of the power supply, the comparingthreshold may be set. For example, without loss of generality, thecomparing threshold is 0.3V.

As such, the difference between the first voltages of the gates of theMOSFETs of the LED string 242-1 and LED string 242-2 is 0.2V, less thanthe threshold 0.3V, so it is believed that the LED string 242-2 does notinclude short circuited LED, and the LED string 242-1 matches the LEDstring 242-2. In contrast, the difference between the first voltages ofthe gates of the MOSFETs of the LED string 242-1 and LED string 242-3 is0.4V, larger than threshold 0.3V, so it is believed that the LED string242-3 includes short circuited LEDs, and the LED string 242-1 does notmatch the LED string 242-3.

First voltage obtaining units 801-1, 801-2 and 801-3 may obtain thecorresponding first voltages by sampling the voltages at the gates ofMOSFETs 241-1, 241-2, 241-3 (e.g., using an analog-digital converter).

Besides, a current arranging unit shown in FIG. 9 may be used to enablethe LED strings 242-1, 242-2, 242-3 all in the same first current.

The current arranging unit 900 differs from the first current obtainingunit 301-1 shown in FIG. 4 in that the first operational amplifier 4122and its relevant connection lines are omitted.

Under the circumstances that the comparing threshold cannot be set, forexample, since the general volt-ampere characteristics of each LED arenot known, it is impossible to detect LED short circuit in a pluralityof LED strings or detect matching among the plurality of LED strings byusing the system 800 as described with reference to FIG. 8.

FIG. 10 illustrates a block diagram of a system for detecting LED shortcircuit in three LED strings shown in FIG. 2 or detecting matching amongthe three LED strings shown in FIG. 2 according to an embodiment of thepresent invention. The system 1000 can address the problem that cannotbe solved by the above system 800.

As shown in FIG. 10, the system 1000 comprises three first voltageobtaining units 1001-1, 1001-2, 1001-3 for obtaining first voltages atthe gates of the corresponding MOSFETs 241-1, 241-2, 241-3 when each ofthe LED strings has same current and the output end of the power supply243 outputs a first voltage;

a changing unit 1002, configured to change the output at the output endof the power supply to a second voltage;

three second voltage obtaining units 1003-1, 1003-2, 1003-3, configuredto obtain second voltages at the gates of the corresponding MOSFETs241-1, 241-2, 241-3 when the output end of the power supply 243 outputsa second voltage;

a determining unit 1004, configured to determine whether there is LEDshort circuit in each of the LED strings or whether each of the LEDstrings matches with each other according to the obtained first andsecond voltages of the gates of the corresponding MOSFETs 241-1, 241-2,241-3.

The first voltage obtaining units 1001-1, 1001-2 and 1001-3 may beidentical with the second current obtaining units 1003-1, 1003-2 and1003-3, and identical with the aforesaid first voltage obtaining units801-1, 801-2 and 801-3.

The second voltage outputted by the output end of the power supply 243may be higher or lower than the first voltage outputted by the outputend of the power supply 243, and the difference between the secondvoltage outputted by the output end of the power supply 243 and thefirst voltage outputted by the output end of the power supply 243 may berelated to for example the number of LEDs for which short circuit is tobe detected. Under the circumstance that the second voltage outputted bythe output end of the power supply 243 is lower than the first voltageoutputted by the output end of the power supply 243, the determiningunit 1004 determines that the LED string whose switch control end hasthe second voltage lower than the maximum voltage among first voltagesof control ends of corresponding switches has LED short circuit ormismatches.

For example, given that the first voltage outputted by the output end ofthe power supply 243 is 66V, the first voltage of the gate of MOSFET241-1 of the LED string 242-1 is 4.5 V, the first voltage of the gate ofMOSFET 241-2 of the LED string 242-2 is 4.3V, and the first voltage ofthe gate of MOSFET 241-3 of the LED string 242-3 is 4.1V; in the casethat the second voltage outputted by the output end of the power supply243 is 60 V, the second voltage of the gate of MOSFET 241-1 of the LEDstring 242-1 is 4.8V, the second voltage of the gate of MOSFET 241-2 ofthe LED string 242-2 is 4.6V, and the second voltage of the gate ofMOSFET 241-3 of the LED string 242-3 is 4.4V. Then, it is determinedthat the LED string 242-3 has LED short circuit or mismatches the LEDstrings 242-1 and 242-2 because its MOSFET gate voltage under a smallersecond voltage outputted by the output end of the power supply 243, issmaller than the voltage of the gate of MOSFET of the LED string 242-1under a larger first voltage outputted by the output end of the powersupply 243. If there is no LED short circuit, this situation issubstantially impossible.

Likewise, when the second voltage outputted by the output end of thepower supply 243 is higher than the first voltage outputted by theoutput end of the power supply 243, the determining unit 1004 determinesthat the LED string whose switch control end has the first voltage lowerthan the maximum voltage among second voltages of control ends ofcorresponding switches has LED short circuit or mismatches.

For example, given that the first voltage outputted by the output end ofthe power supply 243 is 66V, the first voltage of the gate of MOSFET241-1 of the LED string 242-1 is 4.8V, the first voltage of the gate ofMOSFET 241-2 of the LED string 242-2 is 4.6V, and the first voltage ofthe gate of MOSFET 241-3 of the LED string 242-3 is 4.4V; in the casethat the second voltage outputted by the output end of the power supply243 is 72V, the second voltage of the gate of MOSFET 241-1 of the LEDstring 242-1 is 4.5 V, the second voltage of the gate of MOSFET 241-2 ofthe LED string 242-2 is 4.3V, and the second voltage of the gate ofMOSFET 241-3 of the LED string 242-3 is 4.1V. Then, it is determinedthat the LED string 242-3 has LED short circuit or mismatches the LEDstrings 242-1 and 242-2 because the voltage of its switch control endunder a smaller first voltage outputted by the output end of the powersupply 243, is smaller than the voltage of the control end of the switchof the LED string 242-1 under a larger second voltage outputted by theoutput end of the power supply 243. If there is no LED short circuit,this situation is substantially impossible.

The above systems 800 and 1000 may be implemented by using hardware(e.g., control chip), or by using software (e.g., computer programcode), or even by using software in combination with hardware.

FIG. 11 illustrates a flow chart of a method for detecting LED shortcircuit in a plurality of LED strings or detecting matching among theplurality of LED strings according to an embodiment of the presentinvention, wherein one end of each of the plurality of LED strings isconnected to a same output end of a power supply, and the other end isrespectively connected to a corresponding switch.

As shown in FIG. 11, the method 1100 comprises the following steps: step1110 for obtaining first voltages at control ends of the correspondingswitches when each of the LED strings has same current; step S1120 forcalculating differences between the maximum of the first voltages at thecontrol ends of the corresponding switches and other voltages; stepsS1130 for comparing the differences with a comparing threshold; and step1140 for determining the LED strings corresponding to those othervoltages for which differences are larger than the comparing thresholdto have LED short circuit or mismatch with a LED string corresponding tothe maximum voltage.

FIG. 12 illustrates a flow chart of a method for detecting LED shortcircuit in a plurality of LED strings or detecting matching among theplurality of LED strings according to another embodiment of the presentinvention, wherein one end of each of the plurality of LED strings isconnected to a same output end of power supply, and the other end isrespectively connected to a corresponding switch.

As shown in FIG. 12, the method 1200 comprises the following steps: step1210 for obtaining first voltages at control ends of the correspondingswitches when each of the LED strings has same current and the outputend of the power supply outputs a first voltage; step S1220 for changingthe output at the output end of the power supply to a second voltage;step S1230 for obtaining second voltages at the control ends of thecorresponding switches when the output end of the power supply outputs asecond voltage; and step S1240 for determining whether there is LEDshort circuit in each of the LED strings or whether each of the LEDstrings matches with each other according to the obtained first andsecond voltages at the control end of the corresponding switches.

For the purpose of brevity, in the methods as described in FIG. 11 andFIG. 12, it is assumed that the number of LEDs of each LED string is thesame.

According to an embodiment of the present invention, in the methodsdescribed in FIG. 11 and FIG. 12, the corresponding switch isMOSFET/bipolar transistor, and the other end is respectively connectedto the drain of the corresponding MOSFET/a collector of the bipolartransistor, and the control end is the gate of the correspondingMOSFET/a base of the bipolar transistor.

According to an embodiment of the present invention, in the methoddescribed in FIG. 12, the second voltage outputted by the output end ofthe power supply may be higher or lower than the first voltage outputtedby the output end of the power supply. Under the circumstance that thesecond voltage outputted by the output end of the power supply is lowerthan the first voltage outputted by the output end of the power supply,the LED string whose switch control end has the second voltage lowerthan the maximum voltage among first voltages of control ends ofcorresponding switches, has LED short circuit or mismatches; under thecircumstance that the second voltage outputted by the output end of thepower supply is higher than the first voltage outputted by the outputend of the power supply, the LED string whose switch control end has thefirst voltage lower than the maximum voltage among second voltages ofthe corresponding switches, has LED short circuit or mismatches.

According to an embodiment of the present invention, in the methoddescribed in FIG. 11, a magnitude of the comparing threshold correspondsto the number of short circuited LEDs to be detected or to unmatchingstandards.

In any embodiment as described above, a LED also refers to an organicLED (organic light-emitting diode). An OLED a light-emitting diode inwhich the emissive electroluminescent layer is a film of organiccompound which emits light in response to an electric current. Thislayer of organic semiconductor is situated between two electrodes. Inaddition, all or a portion of the switches (e.g., MOSFETs or bipolartransistors) may be either external or internal to a device (e.g., achip or die). In some examples, the switched are integrated into a highvoltage or low voltage device.

What are described are only preferred embodiments of the presentinvention, and not intended to limit the present invention. Thoseskilled in the art appreciate that the present invention may havevarious modifications and variations. Any modifications, equivalentsubstitutes and improvements within the spirit and principles of thepresent invention all fall within the protection scope of the presentinvention.

The invention claimed is:
 1. A system for detecting light emitting diode(LED) short circuit in a plurality of LED strings or detecting matchingamong the plurality of LED strings, wherein one end of each of theplurality of LED strings is connected to a same output end of a powersupply, and the other end is respectively connected to a correspondingdrain/collector of a corresponding switch, and wherein a correspondingsource/emitter of the corresponding switch is connected to ground via acorresponding resistor, the system comprising: a plurality of firstcurrent obtaining units, configured to obtain first currents in the LEDstrings respectively when the output end of the power supply outputs afirst voltage, wherein each of the plurality of first current obtainingunits obtains a corresponding one of the first currents based upon avoltage on a corresponding source/emitter of a corresponding switch; achanging unit, configured to change the output at the output end of thepower supply to a second voltage; a plurality of second currentobtaining units, configured to obtain second currents in the LED stringsrespectively when the output end of the power supply outputs the secondvoltage, wherein each of the plurality of second current obtaining unitsobtains a corresponding one of the second currents based upon a voltageon a corresponding source/emitter of a corresponding switch; adetermining unit, configured to determine, based upon the obtained firstand second currents in the LED strings, whether there is LED shortcircuit in the LED strings or whether the LED strings match one another.2. The system according to claim 1, wherein the switches compriseMOSFETs or bipolar transistors, and said other end is connected todrains of respective MOSFETs or collectors of respective bipolartransistors.
 3. The system according to claim 2, wherein the pluralityof first current obtaining units and the plurality of second currentobtaining units obtain the first currents and second currents ofrespective LED strings in a way of being at one biasing voltage at gatesof the respective MOSFETs or bases of the respective bipolartransistors.
 4. The system according to claim 3, wherein duringobtainment of the first currents and second currents of the respectiveLED strings, the biasing voltage remains constant, and each of the firstcurrent and second current of the respective LED strings is obtained bysampling a voltage at a source of a corresponding MOSFET or an emitterof a corresponding bipolar transistor.
 5. The system according to claim4, further comprising: an analog-digital converter to performanalog-digital conversion for the sampled voltage.
 6. The systemaccording to claim 3, wherein during obtainment of the first currentsand second currents of the respective LED strings, the biasing voltageis controlled to vary, wherein the biasing voltage varies in a way thatthe currents of the LED strings are set, wherein the first current andsecond current of each of the LED strings are respective maximumcurrents among the set currents satisfying the biasing voltage below onebiasing voltage threshold or between two biasing voltage thresholds. 7.The system according to claim 1, wherein the second voltage may behigher than the first voltage or lower than the first voltage; under thecircumstance that the second voltage is higher than the first voltage,the determining unit determines that the LED string having a firstcurrent larger than a minimum second current of the respective LEDstrings includes LED short circuit or mismatch; under the circumstancethat the second voltage is lower than the first voltage, the determiningunit determines that the LED string having a second current larger thana minimum first current of the respective LED strings includes LED shortcircuit or mismatch.
 8. A method for detecting light emitting diode(LED) short circuit in a plurality of LED strings or detecting matchingamong the plurality of LED strings, wherein one end of each of theplurality of LED strings is connected to a same output end of a powersupply, and the other end is respectively connected to a correspondingswitch, and wherein a corresponding source/emitter of the correspondingswitch is connected to ground via a corresponding resistor, the methodcomprising: obtaining, by a plurality of first current obtaining units,first currents in the LED strings respectively when the output end ofthe power supply outputs a first voltage, wherein each of the pluralityof first current obtaining units obtains a corresponding one of thefirst currents based upon a voltage on a corresponding source/emitter ofa corresponding switch; changing output at the output end of the powersupply to a second voltage; obtaining, by a plurality of second currentobtaining units, second currents in the LED strings respectively whenthe output end of the power supply outputs the second voltage, whereineach of the plurality of second current obtaining units obtains acorresponding one of the second currents based upon a voltage on acorresponding source/emitter of a corresponding switch; based upon theobtained first and second currents in the LED strings, determiningwhether there is LED short circuit in the LED strings or whether the LEDstrings match with one another.
 9. The method according to claim 8,wherein the switches comprise MOSFETs or bipolar transistors, and saidother end is connected to drains of respective MOSFETs or collectors ofrespective bipolar transistors.
 10. The method according to claim 9,wherein the first currents and second currents of respective LED stringsare obtained in a way of being at one biasing voltage at gates of therespective MOSFETs or bases of the respective bipolar transistors. 11.The method according to claim 10, wherein during obtainment of the firstcurrents and second currents of the respective LED strings, the biasingvoltage remains constant, and each of the first current and secondcurrent of the respective LED strings is obtained by sampling a voltageat a source of a corresponding MOSFET or an emitter of a correspondingbipolar transistor.
 12. The method according to claim 11, furthercomprising: performing analog-digital conversion for the sampledvoltage.
 13. The method according to claim 10, wherein during obtainmentof the first currents and second currents of the respective LED strings,the biasing voltage is controlled to vary, wherein the biasing voltagevaries in a way that the currents of the LED strings are set, whereinthe first current and second current of each of the LED strings arerespective maximum currents among the set currents satisfying thebiasing voltage below one biasing voltage threshold or between twobiasing voltage thresholds.
 14. The method according to claim 8, whereinthe second voltage may be higher than the first voltage or lower thanthe first voltage; under the circumstance that the second voltage ishigher than the first voltage, it is determined that the LED stringhaving a first current larger than a minimum second current of therespective LED strings includes LED short circuit or mismatch; under thecircumstance that the second voltage is lower than the first voltage, itis determined that a LED string having a second current larger than aminimum first current of the respective LED strings includes LED shortcircuit or mismatch.